Dye-sensitized solar cells (DSSCs) show considerable potential as a relatively low cost alternative to silicon based solar cells. These cells were developed by Gratzel and co-workers in 1991 [B. O'Regan, M. Gratzel, Nature, 353 (1991) 737-740] and there is currently a considerable focus on enhancing their light conversion efficiency and stability.
The principal components of a DSSC electrode are a conducting substrate, which is usually a transparent conductive oxide coated on glass, a highly porous layer of semiconductor material, and a photosensitive dye absorbed into and coating the porous semiconductor.
In the case of conventional DSSCs, dye sensitization involves solely the semiconductor anode made of n-type TiO2 nanoparticles. The counter electrode is generally a metallic cathode with no photoelectrochemical activity. To date the highest conversion efficiency obtained of 11% [M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gratzel, Journal of the American Chemical Society, 127 (2005) 16835-16847], is less than the best silicon based thin-film cells.
A method of further enhancing the light conversion efficiency as suggested by He et al. [J. He, H. Lindström, A. Hagfeldt, S.-E. Lindquist, Solar Energy Materials and Solar Cells, 62 (2000) 265-273] is to substitute the cathode with a dye-sensitized photoactivep-type metal oxide. This tandem dye-sensitized solar cell design utilizes more of the solar spectrum. The efficiency, however of p-type metal oxides is still very low, which limits their effectiveness in tandem DSSCs. Amongst the potential reasons highlighted for the poor conversion efficiency of the cathode within tandem DSSC, the more critical are the inefficient light absorption capability, poor charge injection efficiency and charge transport rate, along with inner resistance.
The most widely used n-type electrode material is nanostructured titanium dioxide. For p-type electrodes, perhaps the most promising technology employs nickel oxide (NiOx) coatings, which has a considerable potential for use as a cathode in tandem cells. This is due to their p-type nature, excellent chemical stability, in addition to well defined optical and electrical properties. Moreover, NiOx is considered as a model semiconductor substrate due to its wide band-gap energy range from 3.6 to 4.0 eV depending on the amount of Ni(III) sites.
NiOx films have been fabricated by various techniques which include spin coating, dipping, electrochemical deposition, magnetron sputtering and sol-gel. With the exception of the sputtering and electrochemical techniques, the other methods require a sintering step in order to obtain dense coatings. Thermal sintering also performs the function of removing the binder in the case of sol gel deposited coatings. Typically sintering conditions of 300-450° C. for 30 to 60 minutes are reported.
A disadvantage with thermal sintering is the processing time. When one adds the heat-up and cool-down times, it can take approximately 4 hours to process a substrate.
Further disadvantages with conventional thermal sintering include the photovoltaic performance of photocathodes produced according to this method and the probably related physical shortcomings of such photocathodes, such as the adhesion between the substrate and the nanoparticular NiOx layer, the post-sintering average particle size, the pore characteristics, and the dye absorption.
The present invention aims to address at least some of these shortcomings and to provide improvements in the manufacture of photovoltaic electrodes.